1. Field of the Invention
The present invention is in the field of electrical power production from steam flashed from high temperature geothermal brines which have high dissolved silica content.
2. Description of the Prior Art
Some high temperature geothermal brine resources are known which contain very large amounts of geothermal energy, but which until recently had not been usable for the commercial production of electrical power because of a high dissolved silica content, which was substantially at the saturation level at source temperatures. Thus, the Salton Sea Known Geothermal Resources Area (KGRA) is estimated to have approximately 3,400 MW.sub.e of geothermal energy available for the generating of electrical power, but until very recently the precipitation of large quantities of dissolved solids, primarily silica, from the brine as its temperature was reduced by the extraction of thermal energy for generating electrical power, has deterred the development of this large geothermal resource. The KGRA geothermal resource is estimated to be a greater energy reserve even than the oil reserves on the north slope of Alaska. Information on this silica precipitation problem has been gathered during independent tests from 1972 to 1976 by Magma Power Company (Magma) and New Albion Resources Co. (NARCO), a subsidiary of San Diego Gas and Electric (SDG&E), and then during operation of the Geothermal Loop Experimental Facility (GLEF) by Magma, SDG&E and NARCO from 1976 to 1979. This testing showed that the silica precipitation problem was so severe that approximately 42 inches per year of scaling, which was primarily amorphous silica, grew on the inner walls of pipes and vessels, and this was impossible or at least uneconomical to deal with by replacing equipment or attempting to physically remove the very hard scale. The severity of this scaling can be appreciated when it is considered relative to typical 10 or 12 inch ID piping, which would become closed to an effectively inoperable status within only a matter of a few weeks.
U.S. Pat. No. 4,302,328 issued Nov. 24, 1981 and U.S. Pat. No. 4,304,666 issued Dec. 8, 1981, both to Van Note, taught the use of a reactor clarifier, a type of apparatus previously known in the sewage treatment art, for the reduction of silica content of the high silica geothermal brines presently under consideration, so as to protect reinjection well equipment against silica plugging. However, this only involved the lower temperature, tail-end part of a proposed geothermal electrical plant, and did not have any effect on the silica precipitation problem in heat extraction apparatus such as steam separators. Thus, where a plurality of steam separators in successively lowering temperature and pressure ranges were contemplated for a geothermal electrical power plant, the use of a reactor clarifier in a downstream, lower temperature location could not check the silica from starting to precipitate out onto walls of a first stage steam separator or from precipitating out in large quantities on walls of a second stage, lower temperature steam separator, as well as in associated conduits and valves.
U.S. Pat. No. 4,429,535 issued to Featherstone Feb. 7, 1984 teaches the use of flash crystallizers for flashing geothermal brine to steam for generating electrical power, and teaches seeding of the hot geothermal brine in the flash crystallizers with seeds derived from a downstream reactor clarifier used as taught in the aforesaid U.S. Pat. Nos. 4,302,328 and 4,304,666, for preferential precipitation of silica upon a vast seed particle area rather than on the surfaces of the flash vessels and associated piping and valves. The Featherstone U.S. Pat. No. 4,429,535 teaches the use of an internal draft tube in each flash crystallizer within which the entering hot geothermal brine is released to flash partially into steam so as to cause recirculation of the seeded brine a plurality of times motivated by the power of the flashing steam within the draft tube. However, such internal draft tube recirculation has several associated problems. One of these is that the brine and flashing steam upwelling through the internal draft tube causes "geysering" or massive entrainment of liquid and particulate silica, and this requires special deflecting equipment to prevent the liquid and particles from passing into the vapor takeoff system. Another problem is that valving of the hot geothermal brine into the flash crystallizers was difficult and scale buildup tended to be a problem in the valve means and associated conduits.
Another prior art method for controlling mineral precipitation in connection with a geothermal electrical power plant was taught in U.S. Pat. No. 3,757,516 issued to McCabe. That patent taught the principle of deep well pumping in the geothermal brine production well and pressurization throughout the entire plant system on through reinjection to avoid loss of the heat of vaporization from that portion of the brine which would otherwise flash to steam in the production well, and incidentally to reduce mineral precipitation in the brine flow path. However, the said McCabe U.S. Pat. No. 3,757,516 was concerned only with those geothermal resources having temperatures under about 400.degree. F., where the loss of heat of vaporization would represent the loss of a considerable proportion of the available thermal energy, so that flashing in the production well bore would cause a serious energy loss in the system. Also, the mineral precipitation that was of principal concern in that patent was calcium carbonate, which, without the pumping, would be precipitated from brines having a substantial calcium oxide content, due to the release of carbon dioxide from the brine during flashing and the chemical combining of carbon dioxide with calcium oxide to form the calcium carbonate precipitate.
The pressurization procedure taught in the McCabe U.S. Pat. No. 3,757,516, while effective to prevent some types of scaling such as from calcium carbonate, would, however, not be effective to prevent dissolved silica from precipitating out on walls of heat extraction and reinjection equipment of a geothermal power plant, as such silica precipitation depends only upon temperature reduction to put the dissolved silica in a supersaturated condition, and time for the slow silica precipitation reaction to occur. Nevertheless, the relatively low geothermal hot water or brine source temperatures to which that patent applied did not carry the large quantities of silica (even if saturated with silica) that are found at substantially the saturation level in very hot brines ranging from about 500.degree. F. to about 620.degree. F. in geothermal energy resources like the Salton Sea and Brawley Geothermal Fields. It is these large quantities of silica in very hot brine which until recently have presented insurmountable problems in attempts to utilize this huge geothermal potential energy resource for the commercial generation of electrical power.
U.S. Pat. No. 4,043,129 issued to McCabe and Zajac applied the deep well pumping concept of the earlier McCabe U.S. Pat. No. 3,757,516 to high temperature geothermal brines above about 500.degree. F. The McCabe-Zajac U.S. Pat. No. 4,043,129 taught that the advantages of deep well pumping, including avoidance of the loss of heat of vaporization and reduction of mineral precipitation, could be realized in connection with very high temperature geothermal brines by mixing a high temperature geothermal brine which might have a relatively high mineral content derived from a relatively deep well with a lower temperature brine that might have a relatively low mineral content derived from a shallower or peripheral well, to produce a brine mixture of sufficiently lowered temperature to be within the practical temperature and pressure ranges for pumping and which may also have a diluted mineral content. However, this still would not solve the serious silica scaling problem in plant and reinjection equipment where the high temperature brines had a dissolved silica content proximate saturation levels at source temperatures, the situation in the Salton Sea and Brawley Geothermal Fields.